1. Field of the Invention
The present invention pertains generally to rubidium frequency standards of the type that use a rubidium vapor lamp as an optical pumping source of a rubidium cell to excite hyperfine transition frequency resonance and more specifically, to an improved method for operating the combination of the rubidium vapor lamp and rubidium cell to provide a more stabilized resonance frequency.
2. Prior Art
Rubidium vapor cell frequency standards are described extensively in the literature. For example, reference may be had to the texts respectively entitled "Frequency and Time" by P. Kartaschoff, Academic Press, 1978; and "Frequency Synthesizers Theory and Design", Second Edition, By Vadim Manassewitsch, John Wiley and Sons, 1980. Such frequency sources are stabilized by quantum mechanical atomic state transition resonances such as the hyperfine atomic resonance frequency related to the change in the internal energy of the atom. The rubidium frequency standard operates as a discriminator, based upon the energy absorption characteristics of rubidium-87. In practice a rubidium lamp passes a lightbeam into a rubidium absorption cell. The rubidium cell absorbs some of the light energy because of the energy level transitions in the rubidium-87 gas. When an electromagnetic field of frequency equal to the resonance frequency of the rubidium vapor is applied to the vapor cell the number of energy level transitions in the rubidium-87 gas is increased and more of the light emitted by the rubidium lamp is absorbed by the rubidium vapor cell. Typically, a photodiode is used to detect the occurrence of the maximum absorption of light from the rubidium lamp which occurs when the frequency of the excitation electromagnetic field exactly matches the rubidium resonance frequency. Typically, a frequency synthesizer is used to generate the appropriate electromagnetic field of frequency of approximately 6,834.685 MHz. This field is modulated at a relatively slow rate (i.e. 154 Hz.) so that the photodiode provides a demodulated signal which may be applied to a phase detector or comparator which also receives the reference modulation signal. The output of the phase comparator is a DC error voltage which is used to control a voltage controlled crystal oscillator at a selected frequency, typically of 5 or 10 MHz. In this manner the frequency of the crystal controlled oscillator is stabilized to approximately one part in 10.sup.11 or better over long periods of time to provide a highly stable and accurate frequency source.
The rubidium-87 resonance frequency is nominally 6.834682614 GHz. plus or minus 3 Hz. However, this frequency is subject to change due to variations in the gas mixture and pressure of the rubidium vapor in combination with well-known buffer gases, as well as temperature variations and magnetic field variations in the cell. Furthermore, the resonance frequency of a rubidium frequency standard is highly dependent upon a shift in the light characteristics of the incident rubidium beam generated by the rubidium vapor lamp.
In co-pending applications assigned to the assignee of the present invention, applicant discloses a number of inventions designed to improve the frequency stability of an atomic clock stabilized crystal oscillator. By way of example, in one such co-pending application, Ser. No. 285,200, filed July 20, 1981, applicant has disclosed a method for operating the rubidium vapor lamp to stabilize the resonance frequency of the standard with variations in temperature of the lamp, and in another co-pending patent application, Ser. No. 285,199, filed July 20, 1981, applicant discloses a novel voltage variable crystal controlled oscillator also having decreased sensitivity to temperature variations.